U.S. patent application number 12/474294 was filed with the patent office on 2010-12-02 for polarized diffractive backlight.
Invention is credited to David James Montgomery, Ioannis PAPAKONSTANTINOU.
Application Number | 20100302798 12/474294 |
Document ID | / |
Family ID | 43220012 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100302798 |
Kind Code |
A1 |
PAPAKONSTANTINOU; Ioannis ;
et al. |
December 2, 2010 |
POLARIZED DIFFRACTIVE BACKLIGHT
Abstract
A backlight is provided for illuminating an at least partially
transmissive display. The backlight includes a light source. A
light guide receives the light from an edge surface and guides the
light by total internal reflection. The light is extracted from the
lightguide using sub-wavelength extraction features designed on the
basis of two interleaved grating structures. The emitted light,
using this arrangement has a high level of polarization.
Inventors: |
PAPAKONSTANTINOU; Ioannis;
(Oxon, GB) ; Montgomery; David James;
(Oxfordshire, GB) |
Correspondence
Address: |
MARK D. SARALINO ( SHARP );RENNER, OTTO, BOISSELLE & SKLAR, LLP
1621 EUCLID AVENUE, 19TH FLOOR
CLEVELAND
OH
44115
US
|
Family ID: |
43220012 |
Appl. No.: |
12/474294 |
Filed: |
May 29, 2009 |
Current U.S.
Class: |
362/601 ;
359/485.05; 362/621; 385/11 |
Current CPC
Class: |
G02B 6/0076 20130101;
G02B 6/0036 20130101; G02B 6/0056 20130101; G02B 6/0038 20130101;
G02F 1/13362 20130101 |
Class at
Publication: |
362/601 ;
362/621; 385/11; 359/486 |
International
Class: |
F21V 7/04 20060101
F21V007/04; G02B 6/00 20060101 G02B006/00 |
Claims
1. A lightguide for distributing light received from a light
source, comprising: a lightguide substrate having first and second
major faces and an edge face there between into which light from
the light source is injected, the lightguide substrate being
configured to transfer the light between the first and second major
faces by total internal reflection; and an extraction feature
structure on at least one of the first and second major faces of
the lightguide substrate to extract the light from the lightguide
substrate, the extraction feature structure including at least a
first grating interleaved with a second grating, wherein a spatial
frequency of the first grating is different than that of the second
grating.
2. The lightguide according to claim 1, wherein the extraction
feature structure is configured to out-couple the light from the
lightguide substrate preferentially one polarization over
another.
3. The lightguide according to claim 1, wherein the spatial
frequency of the first grating is substantially twice that of the
second grating.
4. The lightguide according to claim 1, wherein a feature height of
the first grating is different than a feature height of the second
grating.
5. The lightguide according to claim 4, wherein the feature height
of the second grating alters the height of the first grating.
6. The lightguide according to claim 1, wherein a feature width of
the first grating is different than a feature width of the second
grating.
7. The lightguide according to claim 1, wherein the first and
second gratings are configured such that for a first polarization,
light emitted from the first grating interferes destructively with
light emitted from the second grating and is reflected back into
the lightguide substrate.
8. The lightguide according to claim 7, wherein for a second
polarization different from the first polarization, light emitted
from the first grating does not destructively interfere with light
emitted from the second grating and light of the second
polarization is out-coupled from the lightguide substrate.
9. The lightguide according to claim 1, wherein the at least the
first grating and the second grating interleaved consists only of
the first grating and the second grating interleaved.
10. The lightguide according to claim 1, the extraction feature
structure further comprising at least a third grating interleaved
with the first and second gratings, wherein a spatial frequency of
the third grating is an integer multiple of the spatial frequency
of the first grating.
11. The lightguide according to claim 1, wherein the lightguide
structure comprises first and second layers with light from the
light source being injected at the edge face into the first layer,
the refractive index of the second layer is less than the
refractive index of the first layer, the extraction feature
structure is on the second layer, and the first layer comprises
non-diffractive extraction features that redirect light within the
first layer into the second layer.
12. The lightguide according to claim 11, wherein the lightguide
structure comprises greater than two layers, with at least one of
the layers including the extraction feature structure and another
of the layers including the non-diffractive extraction
features.
13. The lightguide according to claim 1, wherein the first major
face including the extraction feature structure and the second
major face includes a first quarter wave plate layer that rotates
the phase of light incident thereon from within the lightguide
structure without disrupting total internal reflection.
14. The lightguide according to claim 13, further comprising a
second quarter wave plate layer adjacent to but not in optical
contact with the first quarter wave plate layer for correcting
polarization of light reflected back through the first quarter wave
plate.
15. The lightguide according to claim 1, wherein the first and
second gratings comprise symmetric interleaving of at least two
parallel square gratings.
16. The lightguide according to claim 1, wherein the first and
second gratings are lenticular.
17. The lightguide according to claim 1, wherein the first and
second gratings are refractive gratings with no opaque surface
areas.
18. A lightguide for distributing light received from a light
source, comprising: a lightguide substrate having first and second
major faces and an edge face therebetween into which light from the
light source is injected, the lightguide substrate being configured
to transfer the light between the first and second major faces by
total internal reflection; and an extraction feature structure on
at least one of the first and second major faces of the lightguide
substrate to extract the light from the lightguide substrate, the
extraction feature structure including a birefringent diffractive
layer which diffracts unpolarized light from the lightguide
substrate at two different angles corresponding to respective first
and second polarizations.
19. The lightguide according to claim 18, wherein the birefringent
diffractive layer comprises a lenticular square grating.
20. The lightguide according to claim 18, wherein the birefringent
diffractive layer comprises a square array of at least one of a
birefringent, reactive mesogen or liquid crystal material.
21. The lightguide according to claim 18, further comprising a lens
array and a patterned retarder, whereby the lens array is
configured to direct the light of the first polarization through
areas of the patterned retarder different from areas of the
patterned retarder through which the lens array directs the light
of the second polarization to provide light having uniform
polarization.
22. The lightguide according to claim 21, wherein the lens array
comprises a lenticular lens, and the patterned retarder comprises
lenticular strips of birefringent half wave layers that rotate a
plane of polarization by ninety degrees.
23. A backlight for a display device, comprising: a lightguide in
accordance with claim 1; and a light source for providing the light
injected into the lightguide.
24. A backlight for a display device, comprising: at least three
lightguides as set forth in claim 1; a first-colored light source
associated with a first lightguide of the three light guides; a
second-colored light source associated with a second lightguide of
the three light guides; and a third-colored light source associated
with a third lightguide of the three light guides.
25. A display device, comprising: a backlight in accordance with
claim 23; and a spatial light modulator illuminated configured to
be illuminated by the backlight.
Description
TECHNICAL FIELD
[0001] The present invention relates to a backlight, for example
for use with an at least partially transmissive spatial light
modulator. The present invention also relates to a display
including such a backlight. More particularly, the present
invention relates to a distributed illumination panel that may be
used for general illumination.
BACKGROUND OF THE INVENTION
[0002] FIG. 1 of the accompanying drawings illustrates the stack
structure of a typical liquid crystal display (LCD) module of small
size, for example for a mobile phone or PDA device. The display
includes a flat transmissive spatial light modulator (SLM) in the
form of an LCD panel having input and output polarizers on its
bottom and top sides. The rest of the structure is generally
regarded as the backlight system, as follows. A light source (for
example an LED or Laser) emits light, which is coupled into a
lightguide and distributed across the back of the display by way of
total internal reflection (TIR) in such a way that if no scattering
structures were present the light would travel until it reached the
end of the lightguide. Within the lightguide there are multiple
scattering structures that extract the light from the lightguide to
illuminate the LCD panel by disrupting the TIR conditions at the
surface of the lightguide on which they are located, hence allowing
the light to pass through the air-lightguide interface. These
scattering features may be located on either the top or bottom
major lightguide surfaces. The density of the light scattering
features may increase with distance from the light source to
maintain a uniform rate of extraction of the light along the length
of the lightguide. As light is extracted both down and up from the
lightguide, a reflecting film is placed beneath the light guide to
improve the efficiency of the backlight. There are also some
optical films between the lightguide and the LCD panel, placed to
give better illumination uniformity over the display area and to
enhance brightness within a given viewing angle range. These films
typically consist of diffuser layers and prism films that enhance
the central brightness of the backlight. The form of these
structures is well known in the art and will not be discussed
further here.
[0003] The form of the features that extract light from the
lightguide is the main focus of the present invention. The typical
form of extraction features involves "roughening" of the surface in
some manner to disrupt total internal reflection (TIR) in the
lightguide. The extraction in this case produces light that is
emitted at a high angle to the lightguide normal and it preserves
no coherence or polarization of the light.
[0004] There are many types of extraction features that can control
the angle of extraction, for example U.S. Pat. No. 6,786,613
(Minebea) describes wedge shaped extraction features that extract
light in a more vertical direction, but none of these types creates
a polarized emission from an unpolarized source.
[0005] The amount of polarization of any light source is measured
by the ratio of the electric field intensity in two orthogonal
directions. These directions are known as TE (transverse electric)
and TM (transverse magnetic). The ratio of the electric field
intensities is known as the TE/TM ratio and is a measure of the
level of polarization of a beam.
[0006] Conventional art related to polarized emission from a
lightguide guiding unpolarized light is described below.
[0007] The advantage of a polarized backlight is that there is
potentially no loss in the polarizers on the display, this
significantly increasing the brightness of the LC display without
increasing the backlight brightness. A backlight that produces a
TE/TM ratio of substantially greater than 100 would be as good as
the polarizers of the display, making them unnecessary. A backlight
with a lower TE/TM ratio would still improve the loss from the
polarizers.
[0008] Polarization sensitive interference films ("DBEF") that
reflect one polarization and transmit another are well known in the
art. Commercially available versions typically produce a TE/TM
ratio of approximately 3, limited by poor off-axis performance and
absorption losses in the film.
[0009] US 2004/0246743 (Samsung Electronics Co.) describes a
conventional rectangular grating printed on the bottom surface of a
lightguide. The grating exhibits some polarization sensitivity by
out-coupling more of the light belonging to one of the polarization
states, with the polarization ratio of the transmitted field
(TE/TM) depending on the amplitude (height) of the grating.
However, polarization ratio TE/TM is not as high as "DBEF" films.
Also, it works for limited angles of incidence and wavelengths.
[0010] U.S. Pat. No. 5,650,865 (Hughes Electronics) describes a
holographic grating disposed on the top surface of a lightguide
that transmits TE polarization and reflects TM. A phase retarding
film deposited on the bottom lightguide surface gradually converts
TM fields to TE, allowing for polarization recycling. Design is
expensive and difficult to manufacture.
[0011] U.S. Pat. No. 6,688,751 (Slight Optoelectronics Co.)
describes a backlight with a multilayer dielectric film deposited
on its bottom surface. The dielectric film reflects light of one of
the polarization states while it allows through the second state.
Light reflected by the film is out-coupled from the lightguide
while the polarization of the transmitted light is switched as it
passes through a second film so that it can be reused. The design
is expensive and very sensitive to the angle of incidence.
[0012] U.S. Pat. No. 5,258,871 (Eastman Kodak Company) describes a
dual grating which allows for angular separation between TE and TM
polarizations which are subsequently focused onto two different
points. Design is aimed at projectors and not for panel LCDs.
[0013] M. Xu, H. P. Urbach and D. K. G. de Boer, "Simulations of
birefringent gratings as polarizing color separator in backlight
for flat-panel displays," Opt. Express 15, 5789 (2007), describes a
grating made of birefringent material deposited on a lightguide
which transmits TE polarization and reflects TM. The design
exhibits low TE/TM ratio.
SUMMARY OF THE INVENTION
[0014] According to an aspect of the invention, a lightguide is
provided for use in distributing light received from a light
source. The lightguide may be part of a backlight assembly for a
spatial light modular operating on a polarization basis. For
example this could be a liquid crystal display (LCD). The backlight
unit may include a reflecting assembly on the opposite face from
the LCD and not in contact with the lightguide. The light source
may be illuminating another face of the lightguide that may be much
smaller in area than two major faces. A substantial part of the
light is transmitted by total internal reflection across the
lightguide. The lightguide includes at least one layer in
substantial optical contact with adjacent layers. At least one face
or interface between layers consists of a structure that, when
combined, will out-couple preferentially one polarization over
another.
[0015] The structure may include two square gratings interlaced on
the surface, where one grating is substantially twice the spatial
frequency of the other and both gratings have a substantially
different feature width and height, with the higher frequency
grating having the smaller feature width.
[0016] According to another aspect of this invention, another face
of the lightguide or another interface layer that sees totally
internally reflected light in the lightguide includes a quarter
wave plate so that the light that passes through it has its plane
of polarization rotated by 90 degrees.
[0017] In a yet further aspect of this invention, the reflector has
a quarter wave plate on the surface between the reflecting surface
and the lightguide, so that light passing through the quarter wave
plate at near normal incidence has the plane of polarization
rotated by 90 degrees after reflection from the reflector. The
quarter wave plate on the reflector is not in contact with the
lightguide
[0018] The light source could be a LED, fluorescent tube or laser,
for example.
[0019] In a further aspect to this invention, a lightguide is
provided where one surface or interface layer contains a single
grating structure (one or two dimensional, of any shape) that is
constructed from layered birefringent material. External to this
lightguide is also provided a lens array and patterned retarder
layer so that the light leaving the top of the retarder layer is
polarized.
[0020] According to an aspect of the invention, a lightguide for
distributing light received from a light source is provided. The
lightguide includes a lightguide substrate having first and second
major faces and an edge face there between into which light from
the light source is injected, the lightguide substrate being
configured to transfer the light between the first and second major
faces by total internal reflection; and an extraction feature
structure on at least one of the first and second major faces of
the lightguide substrate to extract the light from the lightguide
substrate, the extraction feature structure including at least a
first grating interleaved with a second grating, wherein a spatial
frequency of the first grating is different than that of the second
grating.
[0021] In accordance with another aspect, the extraction feature
structure is configured to out-couple the light from the lightguide
substrate preferentially one polarization over another.
[0022] According to another aspect, the spatial frequency of the
first grating is substantially twice that of the second
grating.
[0023] According to still another aspect, a feature height of the
first grating is different than a feature height of the second
grating.
[0024] In accordance with another aspect, the feature height of the
second grating alters the height of the first grating.
[0025] According to another aspect, a feature width of the first
grating is different than a feature width of the second
grating.
[0026] In accordance with another aspect, the first and second
gratings are configured such that for a first polarization, light
emitted from the first grating interferes destructively with light
emitted from the second grating and is reflected back into the
lightguide substrate.
[0027] According to another aspect, for a second polarization
different from the first polarization, light emitted from the first
grating does not destructively interfere with light emitted from
the second grating and light of the second polarization is
out-coupled from the lightguide substrate.
[0028] According to yet another aspect, the at least the first
grating and the second grating interleaved consists only of the
first grating and the second grating interleaved.
[0029] In accordance with another aspect, the extraction feature
structure further includes at least a third grating interleaved
with the first and second gratings, wherein a spatial frequency of
the third grating is an integer multiple of the spatial frequency
of the first grating.
[0030] According to another aspect, the lightguide structure
includes first and second layers with light from the light source
being injected at the edge face into the first layer, the
refractive index of the second layer is less than the refractive
index of the first layer, the extraction feature structure is on
the second layer, and the first layer includes non-diffractive
extraction features that redirect light within the first layer into
the second layer.
[0031] In accordance with another aspect, the lightguide structure
includes greater than two layers, with at least one of the layers
including the extraction feature structure and another of the
layers including the non-diffractive extraction features.
[0032] According to still another aspect, the first major face
including the extraction feature structure and the second major
face includes a first quarter wave plate layer that rotates the
phase of light incident thereon from within the lightguide
structure without disrupting total internal reflection.
[0033] In yet another aspect, the lightguide further includes a
second quarter wave plate layer adjacent to but not in optical
contact with the first quarter wave plate layer for correcting
polarization of light reflected back through the first quarter wave
plate.
[0034] In accordance with another aspect, the first and second
gratings comprise symmetric interleaving of at least two parallel
square gratings.
[0035] According to another aspect, the first and second gratings
are lenticular.
[0036] According to still another aspect, the first and second
gratings are refractive gratings with no opaque surface areas.
[0037] According to another aspect, a lightguide for distributing
light received from a light source is provided which includes a
lightguide substrate having first and second major faces and an
edge face therebetween into which light from the light source is
injected, the lightguide substrate being configured to transfer the
light between the first and second major faces by total internal
reflection; an extraction feature structure on at least one of the
first and second major faces of the lightguide substrate to extract
the light from the lightguide substrate, the extraction feature
structure including a birefringent diffractive layer which
diffracts unpolarized light from the lightguide substrate at two
different angles corresponding to respective first and second
polarizations.
[0038] In accordance with still another aspect, the birefringent
diffractive layer includes a lenticular square grating.
[0039] According to another aspect, the birefringent diffractive
layer includes a square array of at least one of a birefringent,
reactive mesogen or liquid crystal material.
[0040] In yet another aspect, the lightguide further includes a
lens array and a patterned retarder, whereby the lens array is
configured to direct the light of the first polarization through
areas of the patterned retarder different from areas of the
patterned retarder through which the lens array directs the light
of the second polarization to provide light having uniform
polarization.
[0041] According to another aspect, the lens array includes a
lenticular lens, and the patterned retarder comprises lenticular
strips of birefringent half wave layers that rotate a plane of
polarization by ninety degrees.
[0042] According to another aspect, a backlight for a display
device is provided which includes a lightguide as described herein
and a light source for providing the light injected into the
lightguide.
[0043] In accordance with another aspect, a backlight for a display
device is provided which includes at least three lightguides
described herein; a first-colored light source associated with a
first lightguide of the three light guides; a second-colored light
source associated with a second lightguide of the three light
guides; and a third-colored light source associated with a third
lightguide of the three light guides.
[0044] According to yet another aspect, a display device is
provided which includes a backlight as described herein; and a
spatial light modulator illuminated configured to be illuminated by
the backlight.
[0045] To the accomplishment of the foregoing and related ends, the
invention, then, comprises the features hereinafter fully described
and particularly pointed out in the claims. The following
description and the annexed drawings set forth in detail certain
illustrative embodiments of the invention. These embodiments are
indicative, however, of but a few of the various ways in which the
principles of the invention may be employed. Other objects,
advantages and novel features of the invention will become apparent
from the following detailed description of the invention when
considered in conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 illustrates a conventional backlight of known
type;
[0047] FIG. 2 illustrates an overview of a 1.sup.st embodiment the
present invention;
[0048] FIG. 3a illustrates the detail of the diffractive layer of
the embodiment of FIG. 2;
[0049] FIG. 3b is a graph showing the variations if TE and TM
extracted from the lightguide as a function of grating height for a
particular pitch and incident angle of light.
[0050] FIG. 3c is a graph showing the TE/TM ration as a function of
height.
[0051] FIG. 4 illustrates a 2nd embodiment with multiple layers of
different refractive index;
[0052] FIG. 5 illustrates a 3.sup.rd embodiment with a quarter wave
retarder layer;
[0053] FIG. 6 illustrates a 4.sup.th embodiment with a quarter wave
layer on the reflector;
[0054] FIG. 7 illustrates a 5.sup.th embodiment with retarder
extraction features;
[0055] FIG. 8 is a detailed illustration of the embodiment of FIG.
7;
[0056] FIG. 9 illustrates a 6.sup.th embodiment with three
lightguides and three light sources of different color;
[0057] FIG. 10 illustrates a variation of the embodiment of FIG.
9;
[0058] FIG. 11 illustrates a display device that includes a light
guide in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0059] The present invention will now be described in detail with
reference to the drawings, in which like reference numerals are
used to refer to like elements throughout.
[0060] FIG. 1 illustrates a typical small area liquid crystal
display that is well known in the art. The display includes a
liquid crystal display panel 1 and a backlight unit 2. The
backlight unit 2 includes a number of components that are relevant
to discuss here. A white light source 3, that can be a fluorescent
tube, a LED with phosphor, RGB LED group, a laser or other light
source, is positioned to inject light into the edge of a thin
lightguide 4. The lightguide 4 is designed to transfer by total
internal reflection (TIR) 14 the light across the area of the
display. At least one large or major face, which can be the top,
bottom (illustrated) or both major faces, has features 6 that
disrupt TIR 15 in the lightguide 4 so that the light 13 leaves the
lightguide 4. The light 13 that leaves the lightguide 4 generally
has the wrong angular brightness characteristics for the display,
so four additional layers, a strong diffuser 7, two crossed prism
sheets 8 and 9 in orthogonal directions, and a weak diffuser 10 are
used to produce the correct angular distribution. In many cases the
weak diffuser 10 is incorporated into the top prism layer 9. A
further layer 11 can be a polarization conversion film, which is
typically an interference film that reflects one polarization and
transmits the other. The reflected light is recycled through the
diffusers to become unpolarized and is then reflected by a back
reflector 5 to the film again. The result is that the light is
polarized so that it is better transmitted by the rear polarizer 12
of the liquid crystal SLM 1. The efficiency of this interference
film is limited, however.
[0061] An overview of a first embodiment of the present invention
is shown in FIG. 2.
[0062] The first embodiment of the present invention includes a
backlight 20 that will be described in reference to the
conventional backlight 2 of FIG. 1 and the relevant changes only
will be described here.
[0063] As is shown in FIG. 2, the device in accordance with the
invention includes a liquid crystal spatial light modulator (SLM) 1
and a backlight 20. The backlight includes a light source 3, a
lightguide 4, and a reflector 5. Two sheets, a weak diffuser 21 and
a polarization conversion film 11, are individually possible but
not required in this arrangement. The lightguide 4 has
sub-wavelength extraction features 23 that can be positioned on one
or both major faces of the lightguide 4. In this example, the
extraction features 23 are on the top surface.
[0064] Light 22 in the lightguide 4 is transmitted to the
extraction features by TIR 14 and extracted at the surface 24 by
the extraction features 23. The form of these extraction features
23 is shown in FIG. 3a. Specifically, the extraction features 23
are formed by two or more interleaved gratings. In the present
embodiment, two respective gratings are lenticular and the cross
section is shown in FIG. 3a. The gratings are arranged on the
surface of the lightguide 4 so that the intensity of light emitted
from the lightguide 4 is substantially the same over the display.
This can be achieved by arranging the diffractive features into
areas whose density or sizes alter with position on the lightguide.
The extraction feature cross section includes the symmetrical
interleaving of two parallel square gratings 30 and 31, one grating
having a spatial frequency preferably exactly twice that of the
other. The height 34, 35 and the peak width 32, 33 for the gratings
30 and 31, respectively, are different. The peak width 33 of the
high spatial frequency grating 31 is less than the peak width 32 of
the low spatial frequency grating 30.
[0065] This composite grating represented in FIG. 3a, when incident
with light from a range of angles and polarizations that are
typical in a light illuminated lightguide, emits light in the main
diffractive order that is strongly polarized. The strength of
polarization (TE/TM ratio) and width of the peaks depends on the
conditions of the light illumination, but typically this will be
light within the TIR cone of the lightguide 4. With a single
lightguide 4 and a single wavelength light source 3 such as an LED,
TE/TM ratios greater than 10 are readily possible.
[0066] The grating 23 has no opaque areas on the surface, and is
simply a refractive grating.
[0067] The inventive concept of this invention primarily concerns
the structure of the extraction features as shown in FIG. 3a. More
specifically the design takes advantage of the intrinsic phase
difference that occurs between TE and TM polarizations as they
reflect on the upper and bottom interfaces in a lightguide, 4. The
lenticular grating structure 23 is made from two different
gratings, the second of which alters the height of the first
grating. The effect of this is that the emitted field from one
grating layer is in anti-phase from the field emitted from the
second grating layer for one polarization. Therefore, light of the
first polarization emanating from the two grating layers interferes
destructively and since it cannot transmit through the grating it
all reflects back to the lightguide, 25. However, due to the
intrinsic phase difference between the two polarizations,
interference cannot be destructive for the second polarization
which is allowed to transmit through the grating leading to a well
polarized beam out-coupled from the lightguide.
[0068] FIG. 3b shows the variation of TE, 36, and TM, 37, extracted
from the lightguide as a function of the second grating height 35,
for a particular value of pitch 31, and first grating height 34,
and for a particular incident angle light in the lightguide. FIG.
3c shows the TE/TM ratio as a function of the height 35. At certain
values of height, maxima are seen, 38a and 38b corresponding to the
destructive interference.
[0069] The peaks are dependent on the refractive index of the
lightguide material, the wavelength of the light in the lightguide
and the range of angles in the lightguide. Which peak is used will
depend on a balance on these values.
[0070] An example of this arrangement with a high TE/TM ratio with
a 405 nm LED is as follows. The value of the spatial pitch of the
grating 31 is 155.14 nm, the pitch of the grating 30 is 310.28 nm.
The peak width 33 is 77.1 nm, the peak width 32 is 155.14 nm. The
height 35 is 130.6 nm and the height 34 is 163.3 nm. It should be
noted that these are only examples in a particular case, and that
the invention describes the general shape of multiple gratings
interacting that can be applied to a range of wavelengths,
lightguide shapes and layers.
[0071] This invention should not be limited to simply two gratings.
Multiple gratings that have a pitch that is an integer multiple of
the smallest pitch can be combined to improve the performance of
this system. In the extreme, multiple gratings can be combined to
approximate a continuous curve cross section similar to a discrete
Fourier cosine distribution.
[0072] A second embodiment of the present invention is shown in
FIG. 4. For sake of brevity, only the relevant differences between
this embodiment and the embodiment of FIG. 2 are described
herein.
[0073] In this aspect the lightguide 4 of the backlight 40 has a
second layer 41, in which the refractive index of this layer 41 is
less than that of the lightguide 4. The diffractive features 23 are
placed on the second layer 41. In this arrangement the range of
angles in the second layer 41 is much reduced. This means the
quality of the out-coupled light is significantly improved over the
single-lightguide approach.
[0074] Extraction of light at 42 from the main lightguide 4 into
the secondary lightguide formed by the second layer 41 can be
controlled by appropriate non-diffractive features on the opposite
face of the lightguide 4, for example shallow wedge shaped features
48 that redirect at 47 a small proportion of the lightguide light
45 into the second layer 41.
[0075] The number of such additional layers is not fixed, and they
can be on the lower or both surfaces of the lightguide 4 and any of
the faces or interfaces can have one or more extraction
arrangements.
[0076] In a further embodiment shown in FIG. 5, a modified
lightguide arrangement is suggested. Only relevant differences over
the embodiment of FIG. 2 are described herein for sake of
brevity.
[0077] In the embodiment of FIG. 2, if the extraction is a
significant amount of the light in the lightguide, extraction of
one polarization will reduce the amount of that polarization
relative to the other in the lightguide 4. Thus the assumption of
non-polarized light becomes less true as TE light is extracted, the
TE/TM light ratio in the lightguide 4 reduces, thus extraction
TE/TM further away from the light source 3 will reduce.
[0078] To prevent this, a second layer 50 is added to the face of
the lightguide 4 opposite the extraction features 23. The layer 50
is such that its does not deflect the light (so does not disrupt
TIR), but affects the phase of the incident light at angles to the
normal that are typical to TIR light, such that the plane of
polarization after reflection has rotated by 90 degrees (a quarter
wave plate layer).
[0079] Unpolarized light 52 is incident on the layer of polarized
extraction features 23 that emits TE light 51 from the surface. The
reflected light 53 has a relatively enhanced TM component. The
light is then incident on the quarter wave plate layer 50 and
totally internally reflected 54. The reflected light is rotated to
the TE direction 55 so that extraction for the polarization layer
will then be more efficient and maintain the polarization
out-coupled. The next pass will rotate the plane of polarization
back so that the light in the lightguide 4 is on average
unpolarized, and the light incident on the extraction features 23
is slightly biased towards the preferential TE mode, enhancing
further the extraction efficiency and TE/TM ratio.
[0080] In a further embodiment as shown in FIG. 6 based on the
embodiment of FIG. 5, the nature of the diffractive features is
such that there may be a component of light 61, in the preferential
TE mode directed back into the lightguide 4. Typically this light
will be in a direction that will not be totally internally
reflected by the lightguide 4 and will be extracted by reflection
from the reflector 5.
[0081] In the case where the quarter wave plate layer 50 is at the
bottom of the lightguide 4, the light 61 would pass through as 62
in a circular polarization state, reflected as 64 in a circular
polarization state and will pass through the quarter wave plate
layer 50 to produce light 65 in a TM mode. This will then be
extracted. This light will reduce the final TE/TM ratio of the
backlight.
[0082] This can be removed, as is shown in FIG. 6, by another
quarter wave plate layer 60 positioned on the reflector 5. This
layer 60 is not in optical contact with the other layer 50. The
light passing through the first quarter wave plate layer 50 will be
circularly polarized as 62 but corrected at 63 by the second
quarter wave plate layer 60 to give a circularly rotated beam 64
that will become a TE beam 65 upon passing through the first
quarter wave plate 50 again. This will then contribute to an
improved TE/TM ratio for the system.
[0083] A further embodiment is shown in FIG. 7. A detail on this
embodiment is shown in FIG. 8. This is described with reference to
the embodiment of FIG. 2, but improvements of the subsequent
embodiments can be applied to this embodiment. Referring only to
the relevant differences, the backlight 70 in this embodiment makes
use of a birefringent diffractive layer 71 on the lightguide 4 in
place of the layer of extraction features 23. The birefringent
diffractive layer 71 creates diffraction peaks at two angles for
two different polarizations 86 and 87 from unpolarized light 84
incident on the area 85. The beams are then passed through a lens
layer 72 and then a patterned retarder layer 73 which converts one
of the beams into the opposite polarization state to make a
polarized emission, both beams 74a and 74b are in the same
polarization state.
[0084] This embodiment does not have an issue with the changing
polarization state in the lightguide 4.
[0085] The birefringent layer 71 may include, for example, a
lenticular square grating, patterned as described above, made up of
a square array of birefringent, reactive mesogen or liquid crystal
material 83. In this case "lenticular" refers to line strips
perpendicular to the plane of the page and have the same cross
section along the length. The lens array 72 may include lenticular
lenses 80, and the retarder film 73 may include lenticular strips
81 of birefringent half wave layers that rotate the plane of
polarization by 90 degrees. The retarder area can be made of the
same material as that of the birefringent diffractive area.
[0086] Unpolarized light 84 in the lightguide 4 meets the
diffractive structure 83. The diffractive structure may be the same
structure shape as in FIG. 3a or may be a square grating where the
height 35 is zero. In the case of this embodiment, the features may
or may not be made of the same material as the lightguide, but
would be created of a birefringent material. This means that the
light 84 reaching the grating is diffracted at different angles 86
and 87 according to polarization, because the diffractive nature of
the grating is dependent on the refractive index. The diffraction
split will be in one plane as shown in the diagram, but in a
lenticular form. A lenticular lens 72 collimates the two beams,
where the separation of the lens 72 and the rating plane 71 is
approximately equal to the focal length in the material separating
the layers (e.g. glue or air). The collimation will be spatially
split in terms of polarization, so a second layer 73 above the lens
consisting of lenticular stripes of birefringent material 81, in a
half-wave thickness, is aligned with the lens layer. The pitch of
the stripes is the same as the lens and the width of the stripes is
approximately half the pitch. One polarization is then rotated
producing light emerging from the stripes, 74a and 74b with the
same polarization state.
[0087] All aspects of this invention will work with a white light
source, but a broad wavelength spectrum of the source would not be
optimum for a single design of the extraction films.
[0088] One aspect whereby the polarization state can be improved by
having a coloured source is by mixing different designs that are
optimized for high TE/TM at different wavelengths with the source
spectrum. For example, extraction features optimized for red, green
and blue emission (for example, different values of 30, 31, 32, 33,
34, and 35) can be mixed together rather than using a single mean
design.
[0089] Another aspect is shown in FIG. 9 and is whereby three
lightguides with three red, green, blue light sources 91, 92 and
93, and three designs of extraction features 94, 95 and 96 will
produce a higher TE/TM ratio than the preferred embodiment with a
single lightguide and source.
[0090] Another variation that can be applied to the embodiments
described above is shown in FIG. 10. The display 100 has multiple
colour phosphor layers 101 under internal polarizer structures 102
with the liquid crystal region in the SLM. In this case only one
wavelength, that will excite the phosphors, is necessary for the
backlight to illuminate the panel. The colour emission is made from
the amount of the emitted light 22 passing through the SLM pixels
to the particular phosphor. The extraction features can then be
designed for this wavelength.
[0091] Another variation is shown in FIG. 11. A modified display
111 and backlight component 110 is described with reference to the
embodiment of FIG. 2. This involves a choice of design of
extraction features 112 in place of the extraction features 23 so
that the extracted polarized light 113 is extracted normal to the
lightguide surface. In this case a lens array 114 can be used to
focus light through pixels 115 onto phosphor areas 116 printed on
the outside of the liquid crystal cell 111. The extraction features
112 would be of the same general design as that described in the
preferred embodiment and subsequent embodiments, but may have a
different set of dimensions for the same materials. The phosphors
then produced the colour required 103. The liquid crystal cell need
not then have any colour filters and thus would improve the
brightness of the system. In addition the polarizers 12 and 117 are
standard polarizers, so that this design would be easier to
manufacture.
[0092] The extraction features described here can be manufactured
using nano-imprint techniques that are well known in the prior
art.
[0093] Although the invention has been shown and described with
respect to certain preferred embodiments, it is obvious that
equivalents and modifications will occur to others skilled in the
art upon the reading and understanding of the specification. The
present invention includes all such equivalents and modifications,
and is limited only by the scope of the following claims.
* * * * *